This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Eliminate dependency on io_thread_get().

Obsolete class CloseSongEnumerator, which was a kludge.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Auto-stop the IO thread in all error handlers.

Replaces the method Close().

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Merge some duplicate code.

Avoid GLib.

Obsolete, our logging library doesn't ues GLib anymore.

Allows forward-declaration.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.